The present invention relates generally to solid state lasers and more specifically to a method and apparatus for generating high order harmonics within a laser resonator by inserting several nonlinear crystals.
Although techniques for intra-cavity frequency doubling are well known (for example, see "20 W average-power KTP intra-cavity-doubled Nd:YAG laser" J.O.S.A. Vol 4 No 7, 1987), such techniques produce only second order harmonics.
In a paper entitled "Recent developments in barium borate" by Prof. Chen Chuangtian, a technique is disclosed using a nonlinear crystal outside of a laser resonator to obtain high order harmonic output. The method is shown in FIG. 1 in which (1) is the output coupler mirror of the laser; (5) is the laser active medium, Nd:YAG; (6) is the Q switch; and (7) is the total reflection mirror. Outside of the laser resonator (16), is a frequency doubling crystal (13). If the crystal (14) acts as frequency mixing or quadrupling medium, then third or fourth harmonics could be obtained. However, in practice useful power levels have been successfully obtained only in high peak power pulse laser systems. In a CW Nd:YAG laser, or low peak power pulse laser system, the output of the third or fourth harmonic will be very small when applying the above-referenced methodology, because in low peak power systems, the nonlinear frequency converting process is extremely inefficient, even though no high order harmonic is produced.
It is the principle object of the present invention to overcome the disadvantages set out above, and to provide a method and apparatus which is suitable for producing high order harmonics in CW or low peak power pulse lasers.
The present invention achieves this object by using at least two nonlinear crystals within the resonating laser cavity. By constructing a laser in this way, the nonlinear crystals operate on the fundamental laser beam present within the resonating cavity (which is one to two orders of magnitude higher than the fundamental beam outside of the cavity). By accessing, and operating on, and fundamental laser beam within the resonating cavity, the frequency conversion efficiency is increased, thereby giving rise to the generation of more powerful harmonic frequencies.
According to the invention, nonlinear crystals are inserted in the laser resonator in order to obtain third, fourth or even higher harmonic output. In the system of the present invention, the laser consists of total reflection mirror, output coupler mirror, laser active medium, Q switch and at least two nonlinear crystals. Q switch is located between the laser active medium and the total reflection mirror. There are N nonlinear crystals (N.gtoreq.2) between the laser active medium and the output coupler mirror. Among them, the crystal which is nearest to the laser active medium is a frequency doubling crystal which converts the fundamental wavelength to the second harmonic wavelength. The other (N-1) crystals act as frequency mixing or frequency doubling devices depending on the desired harmonic to be output.
The laser active medium could be, but is not limited to, Nd:YAG, Nd:YLF, Nd:YAP, each of which generates a unique wavelength. When N=2 (meaning that only two crystals are present in the laser cavity), for a fundamental frequency F having wavelength W, the first crystal, which is the nearest to the laser active medium, acts to double the fundamental frequency, and the second crystal acts as a frequency mixing medium. This process is respectively expressed as: EQU F.fwdarw.2F (1) EQU F+2F.fwdarw.3F (2)
When N=3, it denotes that a third crystal is inserted in the laser cavity and acts as a frequency mixer and the process is expressed as: EQU F+3F.fwdarw.4F (3)
The process for a fourth crystal is expressed as: EQU F+4F.fwdarw.5F (4)
Both Type I and Type II phase match methods can be used to realize each of the above processes for frequency doubling and mixing.
Accordingly, for the Nth crystal inserted in the laser cavity, the (N+1)th harmonic is obtained.
Almost any nonlinear crystals can be used in above-mentioned process as frequency doubling and mixing devices, for example, but not limited to, KDP, KD*P, LiNb03, MgO:LiNb03, KNb03, BBO, LBO, and MtiO(X04) (where M is potassium, rubidium, titanium, and X is phosphorous or arsenic). Once the proper crystal is chosen, the working angle for the crystal is worked out according to the corresponding process (1)-(4) and known phase match methods.
In order to further increase the conversion efficiency of above nonlinear process, subresonator mirrors which have high reflectivity for certain harmonic frequency are inserted in the resonator.
One important advantage of the present invention is that the power density of the fundamental wavelength inside the resonator, is typically 1 to 2 times greater than the magnitude outside of the resonator. When a nonlinear crystal is inserted in the resonator, the conversion efficiency of the nonlinear process is greatly increased by the intense fundamental laser beam inside the resonator. By this means, the CW high order harmonic output is realized. This method is also applicable in low peak power pulse laser system to further increase the output of the high order harmonic laser beam.